1. Field of the Invention
This invention relates to an adjustable camera for performing coherent interferometric analysis of object surfaces and more particularly to apparatus for analyzing a variety of surfaces of vehicle tires.
2. Prior Art
The development of the laser as a convenient source of light that is both temporally and spatially coherent has given rise to a class of interferometric techniques for non-destructively testing objects by interferometrically comparing their surfaces before and after the application of a mechanical or thermal stress. For example, subsurface anomalies in rubber vehicle tires may be detected by placing the tires in a pressure chamber and making an interferometric comparison of their surface contours before and after a pressure change. Subsurface separations resulting from improper processing of the tire or impacts experienced during its use will result in non-uniform deformation of the tire surface as a result of the pressure change.
Any of several interferometric techniques may be employed to compare a surface section before and after the pressure change to detect deformation anomalies indicative of subsurface defects. For example, British Patent No. 1,261,542 discloses a method of holographic interferometric analysis of tire defects wherein a double exposure hologram of the tire section formed by making exposures before and after the pressure change may be illuminated to reconstruct an image of the tire section containing interference fringes mapping the surface deformation resulting from the pressure change. Each hologram of the double exposure is made by illuminating photographic film with an "object beam" of coherent light reflected from the tire section and a "reference beam" of light derived from the same coherent light source as the object illuminating beam. This method of deformation analysis has gained wide-spread acceptance for inspection of automotive and aircraft tires.
Alternative interferometric inspection techniques employing double exposures of coherently illuminated surfaces may also be employed for non-destructive testing. U.S. Pat. No. 4,139,302 discloses a double exposure surface analysis method which does not require a separate reference beam but rather passes light from a coherently illuminated surface through a special lens which focuses reflected light into two overlapping and interfering images. This technique has been commercialized under the mark "SHEAROGRAPHY" and substantially relieves the stability requirements associated with holography and accordingly substantially simplifies the analysis mechanism.
The size of the area that can be analyzed by these systems is limited by the power of the laser used and the geometry of the object being analyzed. In systems used to inspect tires it is necessary to take several exposures around the perimeter of the tire. Typically the tire is supported on a horizontal surface, with its central axis vertical, and the interferometric camera is supported centrally within the tire. The tire is rotated relative to the camera between exposures so that successive sectors of the interior of the tire may be analyzed. These cameras are accordingly primarily employed for inspecting the interior crown of the tire. Inspection of the exterior bead and the other side of the lower sidewall is important in certain tires such as aircraft tires which have three beads, one adjacent the inner surface, one adjacent the exterior surface, and one in between. Failures of the tire materials around the external bead are a frequent source of aircraft tire failure and the inability of conventional, centrally mounted, interferometric analysis equipment to inspect these exterior beads and their adjacent sidewall areas limits the effectiveness of the testing technique.